Cryopumps are typically cooled by either open or closed cryogenic cycles and generally follow the same design concept. A low temperature second stage primary cryopanel array, usually operating in the range of 4 to 25 K is the primary pumping surface. This surface is centrally located within a higher temperature housing, usually operated in the temperature range of 60 to 140 K, which provides radiation shielding to the lower temperature primary cryopanel array. The radiation shield is generally closed except at a first stage frontal array positioned between the primary cryopanel array and the process chamber to be evacuated. This higher temperature frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array into the interior of the radiation shield and condense on the primary cryopanel array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the primary cryopanel array may also be provided within the radiation shield to remove the very low boiling point gases such as hydrogen. To prevent overloading of the adsorbent, the adsorbent is generally provided on surfaces which are protected by the primary cryopanel array. By condensing or adsorbing gases onto the pumping surfaces, only a vacuum remains in the process chamber.
In cryopumps where the radiation shield fits closely about the primary cryopanel array, there is limited space between the radiation shield and the primary cryopanel array. In cryopumps of this design, there is a tendency for lower boiling point gases such as argon to condense heavily on the surfaces of the primary cryopanel array closest to the opening through which gases are cryopumped. When this occurs, frost from these condensing gases significantly narrows the gap between the radiation shield and the primary cryopanel array, limiting the ability of other gases to reach the condensing surfaces on the primary cryopanel array further away from the opening as well as the surfaces coated with adsorbent material. A significantly narrowed gap between the radiation shield and the primary cryopanel array greatly reduces the pumping speed of the cryopump.